Workflow question and advice [uv-mapping, modular or singular build?]
I think you ask mainly what is answered on this post which is relevant because it describes when you need different textures and uvs :

Texturing each object separately VS all in the same texture file

generally there is no any rule but it is a common practice to keep things separated depending again on their final usage (gaming, archviz, animation etc), it is more a combination of scale, complexity and purpose.